1. Field of the Invention
The present invention relates to thermoplastic compositions based on polyphenylene ethers.
2. Description of the Background
Polyphenylene ethers are polymers which are characterized by having good thermal and electrical properties. In particular, poly(2,6-dimethyl-1,4-phenylene ether)(herein after, PPE) is a polyphenylene ether which has become industrially important.
Pure polyphenylene ethers are difficult to process due to their high viscosity in the molten state. Additionally, molded pieces produced from pure polyphenylene ethers have high softening temperatures, but poor impact resistance. In particular this poor impact resistance is observed with external notches, indentations, scores and the like.
There have been numerous means proposed for improving the processability and impact strength (the latter as measured by a notched-specimen impact test) of molded or formed pieces comprised of polyphenylene ethers. See e.g., German Pat. No. 1,694,255 (corresponding to U.S. Pat. No. 3,361,851), German Pat. No. 1,694,257 (corresponding to U.S. Pat. No. 3,383,435), and German Pat. No. 1,694,290 (corresponding to U.S. Pat. No. 3,379,792). Despite this prior work, it has been found that the addition of polyolefins, polystyrenes, and/or polyamides still do not optimally modify the properties of PPE-containing molded or formed pieces.
Mixtures of polyphenylene ethers with impact-resistant polystyrenes have become fairly important in industry (see German Pat. No. 2,119,301 and German Pat. No. 2,211,005). These mixtures can be processed into molded pieces having adequante impact strength, but they have the disadvantage that as the content of polystyrenes increases, the softening temperature decreases.
Consequently there has been no dearth of attempts to produce, from pure PPE and rubbers, processible compounds having high softening temperatures (German Pat. No. 2,107,935 and the rejected German OS No. 2,802,664). Apart from the fact that rubber is ordinarily available only in bale form, which cannot readily be metered, it is difficult to mix rubber into PPE with ordinary equipment, since both materials have high viscosities under processing conditions. Further, the result is compositions which are difficult to process in e.g., ordinary injection molding equipment.
It may be seen from the rejected European OS (laid-open document) No. 0,016,829 that in addition to rubbers obtained by the polymerization of butadiene and having the general formula EQU [CH.dbd.CH--(CH.sub.2).sub.2 ].sub.x
there are also now available polyalkenylenes of general formula: EQU [CH.dbd.CH--(CH.sub.2).sub.n ].sub.x
The polyalkenylenes are obtained by ring-opening polymerization of cycloolefins. Here n is the number of ring atoms minus 2.
Polypentenylene is of particular interest, because molding compositions comprised of polyphenylene ethers and polypentenylenes or comprised of polyphenylene ether and a styrene resin modified with polypentenylene yield improved impact resistance. Apart from the fact that polypentenylene is no longer commercially available, it is similarly difficult to mix polypentenylene into polyphenylene ethers as it is to mix ordinary rubbers into polyphenylene ethers. Thus, in all the Examples offered in European OS No. 0,016,829, polystyrene resins or agents to improve processability (e.g., triphenyl phosphate) are employed as additives to the composition. It may be seen from German OS No. 31 38 401 that the molded pieces obtained according to European OS No. 0,016,829 need further improvement in weatherability and impact strength.
In German OS No. 31 38 401, molding compositions containing polyphenylene ethers and an impact resistant modified styrene polymer are claimed. A polyoctenylene with a glass transition temperature of &lt;-40.degree. C. serves as a softening component of the styrene polymer. These mixtures do not yield any improvement in the impact resistance above that afforded by rubbers ordinarily employed as softening components.
Neither compositions based on polyphenylene ethers and polypentenylenes nor those based on polyphenylene ethers and impact-resistant styrene polymer modified with polyoctenylene as a softening component have revealed any particular advantages with respect to impact strength or softening temperature. Polyoctenylene-containing polyphenylene ethers are therefore not at all expected to be suitable for meeting the demanding conditions of high impact strength and simultaneous high softening temperatures of the molding compositions, not to mention the fact that mixing difficulties are posed. There is therefore a distinct need for a polyphenylene ether composition which is characterized by concomitant high impact strength and high softening temperature, and facile processability.